The vast majority of stents for use in the arterial and venous systems have been made by machining a pattern of struts and connecting elements from a metallic tubular preform (typically, by laser machining). Of these machined-tube stents, there have been two basic architectures: circumferential and helical. Circumferential configurations are based upon a series of cylindrical bands joined longitudinally by bridges to make a tubular structure. Helical configurations include a continuous helical structure (typically made of an undulating pattern of struts and end-loops) with joining structures (referred to as “bridges”) joining adjacent turns of the helix to provide mechanical integrity to the tubular structure (to prevent unwinding, kinking, and buckling.
Stents have been developed for use in various lumens of the body, including the biliary tree, venous system, peripheral arteries, and coronary arteries. Stents are used to open or hold open a lumen that has been blocked (occluded) or reduced in size (stenosed) by some disease process, such as atherosclerosis or cancer. Previously developed stents for use in the biliary, venous, and arterial systems have been of two broad classes: balloon-expanded and self-expanding. In both of these classes, stents have generally been made by two different techniques: either formed from wire or machined from a hollow tube. Other manufacturing techniques have been proposed, such as vacuum or chemical deposition of material or forming a tube of machined flat material, but those “exotic” methods have not been widely commercialized.
Fine Cell Structure of Stents
Clinicians recommend the use of stents with relatively small openings to minimize the chances of friable material from the lumen wall penetrating into the interior of the stent where it may result in narrowing of the lumen by cellular proliferation or where it may embolize downstream, causing damage or ischemia. U.S. Pat. No. 6,537,310 to Palmaz et al. teaches that it is advantageous to cover a stent with a porous film having openings no larger than 17 microns in their smallest dimension to minimize the migration of embolic debris and plaque into the lumen of a stent. However, Palmaz teaches use of a stent that is very difficult to manufacture because of the great number of very small openings in the covering film or “web.”
Clinicians have asked for stents with “thin, equi-spaced struts for optimal wall coverage and drug elution” (“Clinical Impact of Stent Design: Results from 10 Years Experience,” C. DiMario, TCT2003). DiMario demonstrates 15.0% restenosis versus 36.6% for stents with thin struts (50 microns, Multilink) versus thick struts (average of all stents evaluated with struts greater than or equal to 100 microns). DiMario also relates stent efficacy to “integrated cell size,” showing better results for the BX VELOCITY® stent with cells of 3.3 mm2 versus stents with larger cell sizes. DiMario reports reduced neointimal hyperplasia for smaller struts (0.8 mm thickness for closely-spaced 125-micron struts versus 1.54 mm thickness for wider-spaced 200-micron struts). Because prior art stent designs have large gaps between stent parts, drug elution about these parts does not adequately cover all of the tissue within the bounds of the stent.
In “Clinical Impact of Stent Design: Results From Randomized Trials” (TCT 2003), A. Kastrati reports reduced residual percent-diameter stenosis after stenting (4.0% versus 5.7%) with 50-micron struts (Multi-link) versus 140-micron (Multi-link Duet).
In his report “Era of Drug-Coated and Drug-Eluting Stents” (TCT 2002), G. Grube states that the typical open-cell configuration gives poor distribution of the drug into the arterial wall because of the large open gaps when the stent is situated in a bend of the artery.
Number-of-Struts to Strut-Length Ratio
U.S. Pat. No. 6,129,755 to Mathis et al. (hereinafter “Mathis”) teaches improved self-expanding stents with circumferential hoops of struts joined by oblique longitudinal bridges described therein is the importance of having a large number of struts per hoop (the number of struts counted by going around the circumference) and minimum strut length to minimize strains in superelastic materials and to prevent emboli from passing through the wall of the stent. Mathis defines a figure of merit that is the ratio of number of struts around the circumference to the length (in inches) of a strut, measured longitudinally. This ratio, which has the units of reciprocal inches, will be referred to herein as the M-D Ratio because the inventors were Mathis and Duerig. Mathis describes prior-art stents as having a ratio of about 200 and that their improved stent has an M-D Ratio of over 400. A representative stent produced by Cordis Corporation according to the Mathis-Duerig invention—referred to as the “SmartStent”—has 32 struts per circumference and shut lengths of approximately 0.077 inch, resulting in an M-D Ratio of approximately 416.
The M-D Ratio is determined by number of struts divided by strut length. For a given diameter stent, assuming “maximum-metal” configuration, which is typical for self-expanding: stents, the number of struts around the circumference is inversely proportional to the strut width. Thus, the M-D Ratio is inversely proportional to the product of strut width and length.